1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly to a method and apparatus of driving a liquid crystal display that is adaptive for reducing the number of frame memories.
2. Description of the Related Art
A liquid crystal display device controls the light transmissivity of liquid crystal cells in accordance with a video signal, thereby displaying a picture.
In the liquid crystal display device, an active matrix liquid crystal display device in which a switching device is formed at each liquid crystal cell better displays a motion picture because active control of the switching device is possible. A thin film transistor (hereinafter, referred to as “TFT”) is mainly used as the switching device in the active matrix liquid crystal display.
However, the response speed of liquid crystal display, as shown in mathematical formulas 1 and 2, and dependent on the unique viscosity and elasticity of liquid crystal.
                              τ          r                ∝                              γ            ⁢                                                  ⁢                          d              2`                                            Δɛ            ⁢                                                                          V                  a                  2                                -                                  V                  F                  2                                                                                                      [                  MATHEMATICAL          ⁢                                          ⁢          FORMULA          ⁢                                          ⁢          1                ]            
Herein, τr represents the rise time when a voltage is applied to liquid crystal, Va represents the applied voltage, VF represents the Freederick Transition Voltage where a liquid crystal molecule starts to tilt, d represents the cell gap of the liquid crystal cell, and γ (gamma) represents the rotational viscosity of the liquid crystal molecule.
                              τ          f                ∝                              γ            ⁢                                                  ⁢                          d              2                                K                                    [                  MATHEMATICAL          ⁢                                          ⁢          FORMULA          ⁢                                          ⁢          2                ]            
Herein, τf represents the fall time when the liquid crystal is restored to its original location by its elastic restitutive force after the voltage applied to the liquid crystal is turned off, and K represents the unique elastic modulus of liquid crystal.
The response speed of a twisted nematic TN mode liquid crystal is generally used may differ in accordance with the physical properties and cell gap of the liquid crystal material, but conventionally, the rise time is 20˜80 ms and the fall time is 20˜30 ms. In this case, the response speed of the liquid crystal is longer than one frame period (NTSC: 16.67 ms). Because of this, it progresses to the next frame before the voltage charged in the liquid crystal cell reaches a desired voltage, as shown in FIG. 1, thus motion blurring is generated in which the screen gets blurred when playing the motion picture.
Referring to FIG. 1, a liquid crystal display device of the prior art does not express a desired color and brightness because the display brightness BL corresponding thereto does not reach the desired brightness when the data VD is changed from one level to another level. As a result, the liquid crystal display device is blurry when showing a motion picture, and its picture quality drops due to the deterioration of the contrast ratio.
In order to solve the slow response speed of the liquid crystal display device, U.S. Pat. No. 5,495,265 or PCT International Publication No. WO99/05567 has suggested a method of modulating data in accordance with the existence or absence of the change of the data using a look-up table, hereinafter referred to as a “high-speed driving method”. The high speed driving method modulates the data using the principle shown in FIG. 2.
Referring to FIG. 2, the high speed driving method modulates input data VD into a pre-set modulated data MVD, and the modulated data MVD is applied to the liquid crystal cell to get the desired brightness MBL. The high speed driving method has a value of |Va2−VF2| in Mathematical Formula 1 on the basis of the existence or absence of a change of the data in order to get a desired brightness corresponding to the brightness value of the input data within one frame period. Accordingly, the liquid crystal display device using the high speed driving method compensates for the slow response time of liquid crystal by modulating the data value to ease blurring of the motion in a motion picture.
In other words, the high speed driving method modulates the data of the current frame to a pre-set modulated data if there is any change between the data when the data are compared between the previous frame and the current frame. A high speed driving apparatus in which the high speed driving method is realized is shown in FIG. 3.
Referring to FIG. 3, the high speed driving apparatus includes first and second frame memories 33A, 33B to store the data from a data bus 32, and a lookup table 34 to modulate the data.
The first and second frame memories 33A, 33B alternately store the data of the frame in accordance with a pixel clock, alternately output the stored data, and supply the previous frame data, i.e., (n−1)th frame data (Fn−1), to the lookup table 34.
The look-up table 34 selects a pre-set modulated data MRGB by having the nth frame data (Fn) and the (n−1)th frame data (Fn−1) from the first and second frame memory 33A, 33B as the address, thereby modulating the data. The lookup table 34 includes a read only memory ROM and a memory address control circuit.
TABLE 101234567891011121314150023456791012131415151515101345678101213141515151520024567810121314151515153001356781011131415151515400134678911121314151515500123578911121314151515600123468910121314151515700123457910111314151515800123456810111214151515900123456791112131415151000123456781012131415151100123456789111314151512001234567891012141515130012334567810111315151400123345678911121415150001233456789111315
In the Table 1, the leftmost column represents the data of the previous frame Fn−1 and the uppermost row represents the data of the current frame Fn.
For the nth frame period, the nth frame data (Fn) is stored at the first frame memory and supplied to the lookup table 34 in accordance with the same pixel clock as shown by the solid lines. At the same time, the second frame memory 33B supplies the (n−1)th frame data (Fn−1) to the lookup table 34.
Differently from this, for the (n+1)th frame period, the current (n+1)th frame data (Fn+1) is stored at the second frame memory 33B and simultaneously supplied to the lookup table 34 in accordance with the same pixel clock as shown by the dotted lines. At the same time, for the (n+1)th frame period, the first frame memory 33B supplies the nth frame data (Fn) to the lookup table 34.
In this way, the high speed driving apparatus needs two frame memories 33A, 33B in order to alternately supply the previous frame data to the lookup table 34. However, frame memories are expensive. As the use of multiple frame memories increases the circuit cost, a method of reducing the number of frame memories and reducing the capacity is desirable.